Inductive element for intravascular implantable devices

ABSTRACT

An inductive element adapted for use in implantable intravascular devices (IIDs) having an elongate form factor with a cross-section. The inductive element includes a core that has an outer surface contour that corresponds to the form factor. A set of elongate, or oblong, windings are situated lengthwise along the major length dimension of the inductive element. The windings are also situated to direct a magnetic field along a radial direction in relation to the elongate form factor. In one embodiment the form factor is generally cylindrical and the cross-section is generally round.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/833,101now U.S. Pat. No. 8,060,218, filed Aug. 2, 2007, the disclosure of whichis hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to electrical components and,more particularly, to an inductive element, such as a choke ortransformer, that has a narrow form factor suitable for use inimplantable medical devices such as intravascular devices.

BACKGROUND OF THE INVENTION

Implantable medical devices such as pacemakers, defibrillators, andimplantable cardioverter defibrillators (“ICDs”) have been successfullyimplanted in patients for years for treatment of heart rhythmconditions. Pacemakers are implanted to detect periods of bradycardiaand deliver low energy electrical stimuli to increase the heart rate.ICDs are implanted in patients to cardiovert or defibrillate the heartby delivering high energy electrical stimuli to slow or reset the heartrate in the event a ventricular tachycardia (VT) or ventricularfibrillation (VF) is detected. Another type of implantable devicedetects an atrial fibrillation (AF) episode and delivers electricalstimuli to the atria to restore electrical coordination between theupper and lower chambers of the heart. Still another type of implantabledevice stores and delivers drug and/or gene therapies to treat a varietyof conditions, including cardiac arrhythmias. The current generation forall of these implantable devices are typically can-shaped devicesimplanted under the skin that deliver therapy via leads that implantedin the heart via the patient's vascular system.

Next generation implantable medical devices may take the form ofelongated intravascular devices that are implanted within the patient'svascular system, instead of under the skin. Examples of theseintravascular implantable devices are described, for example, in U.S.Pat. No. 7,082,336, U.S. Publ. Appl. Nos. 2005/0043765A1, 2005/0208471A1and 2006/0217779A1. Devices of this type can have a diameter of about3-15 mm and a length of about 10-60 cm to facilitate insertion andimplantation inside of the vasculature, while permitting a sufficientamount of blood flow around the device. Within geometric constraintssuch as these, the devices contain electrical/electronic components andcircuitry for performing their various functions.

Implantable devices have on-board energy storage (typically, batteries),and high voltage converter circuits for converting the stored energyinto a form suitable for operating the device to deliver electrotherapytherapy. In cardioverter/defibrillator-type devices, the high voltageconverter circuitry typically includes a circuit that produces energy athigh voltage (typically at least in the range of 50-800 Volts and 1-40Joules) for use in the application of the cardioversion/defibrillationelectrotherapy. Because there is only a finite amount of energyavailable in the energy storage, and because replacing the batteriestypically involves a surgical procedure to remove or otherwise accessthe implanted device or a recharge process that can require extendedperiods of time for recharging the energy storage, providing highlyefficient circuitry is important to prolonging the useful life of thedevice and also to making the device as small as practicable.Accordingly, the high voltage converter circuit used in implantabledevices should be as efficient as possible.

A switching mode power converter is generally considered to be one ofthe most efficient arrangements for stepping up voltage from the energystorage to the high voltage required for delivery of the electrotherapy.This type of converter operates by applying intermittent current to aninductive element such as a choke or a transformer, and harnessing thevoltage-boosting effect produced by the associated time-varying magneticfield generated by the inductive element. A variety of switchingconverter topologies and operating modes are well-known. Examplesinclude the boost converter, the flyback converter, the SEPIC(single-ended primary inductance converter), and the Cuk converter. Theboost converter and certain Cuk converter topologies use one or moreinductors, whereas the flyback, SEPIC, and other types of Cuk convertersuse transformers as the principal inductive elements for performing thevoltage conversion function. Certain SEPIC topologies use both, aninductor, and a transformer.

The inductive element (whether an inductor or a magnetically coupled setof inductors) is generally constructed from at least one coil of wireand a magnetic core of high relative permeability material, such asferromagnetic material. The core operates to confine the magnetic fieldclosely to the element, thereby increasing its inductance. The coreprovides a magnetic flux path that guides the flux through the center ofthe coil(s) and along a return path that can be contiguous, or canalternatively have a plurality of non-contiguous return path portions. Avariety of core geometries are known for inductive elements. Some areconstructed as enamel coated wire wrapped around a ferrite bobbin withwire exposed on the outside, while others enclose the wire completely inferrite for improved shielding effect. Core geometries typically includetoroidal structures, C- or E-shaped structures, pot-shaped structuresand planar structures.

In the case of a switching mode transformer, a typical turns ratio foruse in a high voltage converter circuit for an implantable device can beon the order of Np:Ns being 1:15, where Np is the number of primaryturns and Ns is the number of secondary turns. Unlike transformers usedfor signals and linear power supplies, transformers used in switchingmode circuits are designed not only to transfer energy, but also tostore the energy for a significant fraction of the switching period. Forinstance, in a power converter switching at about 60 kHz (which is afrequency selected to keep core eddy current losses low) and having atransformer with a core made from a power ferrite material with relativepermeability of 2000 to 4000, a certain minimum primary inductance isrequired in the transformer.

Most of the stored energy in an inductive element is stored in an airgap of the core. A certain air gap volume is needed to store the desiredenergy. However, increasing the gap length reduces the inductance in thetransformer or inductor. Winding inductance in an inductive element isdirectly proportional to the square of the number of windings, and tothe magnetic cross sectional area orthogonal to the direction ofmagnetic flux produced in the volume. To compensate for the loss ofinductance due to an increased air gap, a greater number of windings ora greater cross-sectional area for the magnetic flux path is needed.More windings take up more volume, and increase the power losses in thedevice due to increased resistance. Increasing the cross-sectional areafor the magnetic flux path in a conventional core geometry would involveincreasing the size of the core and consequently taking space away fromthe windings or increasing the overall size of the device.

In terms of an intravascular implantable device which may take the formof an elongated structure implanted within a patient's vasculature andgenerally having a circular cross-sectional area, if a standard circularpot core is used as the ferrite core of the transformer, the magneticcross sectional area will be limited to something less than thecross-sectional area of the implantable device. Given this limitation,one alternative to increasing inductance is to increase the number ofwindings. Unfortunately, this adds to the winding volume in thetransformer as a relatively high windings turns-ratio is needed for thehigh voltage converter. Aside from the higher overall resistance in thewindings by increasing the total number of turns, this approach wouldalso require a longer transformer to accommodate the windings.

A long and narrow pot core poses difficult winding challenges when usedin an implantable intravascular device owing to the limited windingcross sectional area across the diameter of the core. Furthermore, thereis a practical limit to the length of the transformer in implantableintravascular devices. For instance, the housing of the implantableintravascular device must provide a certain amount of flexibility tofacilitate routing of the device through the vasculature. Longersections of rigid housing elements limit the flexing radius of thedevice. In addition, the enclosure section housing the transformer mayneed space beyond the ends of the transformer to house circuitry,input/output hardware, wiring, and the like.

Other approaches, such as scaling down an E core or one of itsderivatives, such as the EFD or ER cores, for use within the dimensionalconfines of an intravascular device may not be feasible given the energystorage and inductance requirements for the power converter circuit. Forinstance, there may be insufficient winding area to achieve the targetprimary inductance for a transformer. Even if the electrical performancewere achievable in the small size, using a scaled-down E core-typeinductive element in the intravascular device's housing would bewasteful of housing volume because excess volume would remain in thehousing around the inductive element.

Given the size constraints of intravascular implantable devices,designing a power converter that can effectively and efficientlygenerate the high voltage electrotherapy signals using present-dayinductive elements presents significant challenges. Typical core shapesand geometries, such as the E, C, toroidal, and pot cores ordinarilycapable of providing the required functional and performancerequirements for high voltage converters in conventional implantabledevice like conventional can-shaped implantable defibrillators are notwell-suited for use in the small-diameter space of implantableintravascular devices.

SUMMARY OF THE INVENTION

The present invention is generally directed to an inductive elementadapted for use in implantable intravascular devices (IIDs) having anelongate form factor adapted for implantation in the vasculature. Theinductive element includes a core that has an outer surface contour thatcorresponds to an interior surface contour of a form factor of the IID.A set of elongate, or oblong, windings are situated lengthwise along themajor length dimension of the inductive element. The windings are alsosituated to direct a magnetic field along a radial direction in relationto the longitudinal axis of the form factor of the IID.

In one aspect of the present invention, an implantable intravascularmedical device includes a structure that defines a form factor having anelongate geometry including a length and a generally roundcross-section, the cross-section being defined perpendicularly to thelength. One example of such a structure is a housing, or a portion of anenclosure that provides a hermetic barrier, and has a generallycylindrical form factor suitable for implantation in the vasculature. Acircuit is situated within the form factor and includes an energystorage device, such as a battery, and a converter circuit that operatesto convert an output of the energy storage device into a relativelyhigher voltage. The converter circuit includes an inductive element thathas an outer surface of a shape that corresponds to the form factor. Theinductive element has a coil positioned to direct a magnetic fieldgenerally perpendicularly to the length.

An implantable intravascular medical device according to another aspectof the present invention includes a structure that defines a form factorhaving an elongate geometry including a form factor length and agenerally round form factor cross-section defined perpendicularly to theform factor length. A circuit is situated within the form factor and hasan inductive element, which includes a core of magnetic material havinga core length and a core cross-section defined perpendicularly to thelength, and a coil having a plurality of windings that define a looparea. A portion of the core is situated in the loop area such that thecoil, when energized, produces a magnetic flux in the core along aforward path and a return path. A sum of a total cross-sectional area ofthe magnetic flux in the forward path and a total cross-sectional areaof the magnetic flux in the return path is greater than an area of thecore cross-section.

An inductive element (e.g., an inductor or transformer) according to oneaspect of the invention includes a core of magnetic material having acore length and a generally cylindrical outer boundary, at least onecoil having a plurality of windings that define a loop area. A portionof the core is situated in the loop area such that the at least onecoil, when energized, produces a closed magnetic flux along a flux paththrough the core. The length of the flux path is less than the corelength.

According to another aspect of the invention, an inductive element foruse in an implantable intravascular device comprises a core of magneticmaterial. The core has a major longitudinal dimension along a firstreference axis and a core cross-section having a generally round outerboundary, with the core cross-section being defined in a first referenceplane that is orthogonal to the first reference axis. The core includesa post, and at least one coil is arranged around the post, such that thecoil is situated to direct a magnetic field perpendicularly to the firstreference axis when energized.

A method of making an implantable intravascular device according toanother aspect of the invention involves forming a generally hermeticbarrier for enclosing a circuit, with the barrier having a generallycylindrical exterior surface and defines an interior form factor. Aninductive element is assembled as part of the circuit to be situatedwithin the barrier such that at least a majority of an outer surfacecontour of the inductive element corresponds to the interior formfactor. To this end, a set of elongate windings are situated lengthwisein the barrier to direct a magnetic field along a radial direction inrelation to the barrier, and a closed magnetic path is providedsubstantially through a permeable material for the magnetic field.

In one example embodiment, the inductive element has a cylindrical outerwall that matches the cylindrical inner wall of a compartment or otherenclosure portion housing the components. In another example embodiment,the inductive element has a cylindrical outer wall that has dimensionswithin predefined constraints related to at least a portion of anexterior IID surface formed around the inductive element. Assembling theinductive element includes situating a set of elongate windingslengthwise in the compartment to direct a magnetic field along a radialdirection in relation to the compartment; and providing a closedmagnetic path substantially through a permeable material for themagnetic field. The closed magnetic path can be provided by providing amagnetic core with or without an air gap.

The approach taken by embodiments of the invention provides inductiveelements that improve the volume within the form factor of an IIDavailable for the magnetic material, while providing a relatively largerand more usable magnetic flux cross-sectional area for improvedinductance and lowered AC flux density. Improving the usable volume canbe accomplished by shaping much of the transformer contour to fit in agenerally cylindrical space associated with the form factor. Endowing alarge cross sectional area generally orthogonal to magnetic flux can berealized by winding the transformer conductors on a plane lengthwise tothe IID. The direction of magnetic flux generated through the crosssectional area formed by the winding is along an axis perpendicularthereto. The relatively large magnetic flux can be useful in certainpower converter topologies such as, without limitation, the flyback,SEPIC, or Cuk converters. The inductive element may also be used inother types of power circuits, such as a buck or boost regulator, orother circuits utilizing inductors or transformers.

The form factor according to certain embodiments of the invention can bedefined based on the IID housing dimensions, and on the presence ofother components within the enclosure portion housing the inductiveelement. For instance, in embodiments where additional electrical ormechanical components such as wiring, interface hardware, or circuitryis to be present in the housing in which the inductive element issituated, the form factor can take the volume constrained by thesecomponents and housing into account. In a related type of embodiment,the form factor can include space along the length of the transformerfor wiring or circuitry running lengthwise past the inductive element.

Aspects of the invention enable the circuitry of an IID to achievelevels of performance in power converter circuits, among other types ofcircuits, that occupy the confined space of IIDs, levels which werepreviously unattainable using conventional power converter components inthe same dimensional constraints. These advances can lead to the designof smaller and higher-performing implantable intravascular devices thatare advantageously easier to implant in patients and administer moreeffective electrotherapy with a longer service life compared to devicesbased on conventional technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a perspective illustration depicting human cardiac anatomy.

FIG. 2A is a plan view of an implantable intravascularelectrophysiologic therapy device according to one embodiment of thepresent invention.

FIG. 2B is a schematic representation of the implantable device FIG. 2A.

FIG. 3A is a plan view of an implantable intravascularelectrophysiologic therapy device according to another embodiment of thepresent invention.

FIG. 3B is a schematic representation of the implantable device FIG. 3A.

FIGS. 4A-4E are circuit diagrams illustrating various known types ofswitching regulators topologies.

FIGS. 5A-5G are a perspective view diagrams of various inductive coresof known geometries.

FIG. 6 is an exploded view diagram illustrating a narrow form factorinductive element assembly according to one aspect of the invention.

FIG. 7A is an exploded view diagram illustrating another narrow formfactor inductive element assembly according to another aspect of theinvention.

FIG. 7B is a cross-sectional view of the assembled inductive element ofFIG. 7A.

FIG. 8 is a diagram illustrating simulated magnetic flux densitythroughout the core of an exemplary inductive element according to oneembodiment of the invention, such as the inductive element of FIG. 6.

FIG. 9 is a diagram illustrating simulated magnetic flux densitythroughout the core of an exemplary inductive element according toanother embodiment of the invention, such as the inductive element ofFIGS. 7A-7B.

FIGS. 10 and 10A and 10B is a diagram illustrating a power convertercircuit and a portion of a control system for the power converteraccording to one example embodiment.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-known methods,procedures, and components may not been described in detail so as to notunnecessarily obscure aspects of the present invention.

Referring now to FIG. 1, the general cardiac anatomy of a human isdepicted, including the heart and major vessels. The following anatomiclocations are shown and identified by the listed reference numerals:Right Subclavian 102 a, Left Subclavian 102 b, Superior Vena Cava (SVC)103 a, Inferior Vena Cava (IVC) 103 b, Right Atrium (RA) 104 a, LeftAtrium (LA) 104 b, Right Innominate/Brachiocephalic Vein 105 a, LeftInnominate/Brachiocephalic Vein 105 b, Right Internal Jugular Vein 106a, Left Internal Jugular Vein 106 b, Right Ventricle (RV) 107 a, LeftVentricle (LV) 107 b, Aortic Arch 108, Descending Aorta 109, RightCephalic Vein 109 a (not shown in FIG. 1), Left Cephalic Vein 109 b,Right Axillary Vein 110 a (not shown in FIG. 1) and Left Axillary Vein110 b.

One embodiment of the present invention describes intravascularelectrophysiological systems that may be used for a variety of functionsto treat cardiac arrhythmias with electrical stimulation. Thesefunctions include defibrillation, pacing, and/or cardioversion. Ingeneral, the elements of an intravascular implantable device forelectrophysiological therapy include at least one device body andtypically, but optionally, at least one lead coupled to the body.Alternatively, the intravascular implantable device may have no leads,such as for an embodiment of an intravascular implantable drug/genetherapy device, a combination intravascular implantable device that candeliver both electrical therapy and/or drug/gene therapy, or anotherintravascular implantable device in which a high voltage convertercircuit is utilized to, for example, power drug/gene therapy deliverydevices/pumps or electrically powered delivery/therapy devices.

Various examples of intravascular implantable electrophysiology devices,such as intravascular defibrillation and/or pacing devices 20 and leads28 will be given in this description. In those examples, referencenumerals such as 20 a, 20 b, 20 c, etc., will be used to describecertain embodiments of the intravascular device 20, whereas elsewherereference numeral 20 may be used to more generally refer tointravascular devices of the type that may be used with the presentinvention for providing therapy other than, or in addition to, cardiacelectrophysiology. Likewise, reference number 28 may be used generallyto refer to leads of a type that may be used with one embodiment of thesystem. Reference number 100 refers generally to vessels and/or vesselwalls within the human body.

In one embodiment, device 20 includes components, known in the art to benecessary to carry out the system functions of an implantableelectrophysiology device. For example, device 20 may include one or morepulse generators, including associated batteries, capacitors,microprocessors, and circuitry for generating electrophysiologicalpulses for defibrillation, cardioversion and/or pacing. Device 20 mayalso include detection circuitry for detecting arrhythmias or otherabnormal activity of the heart. The specific components to be providedin device 20 will depend upon the application for the device, andspecifically whether device 20 is intended to perform defibrillation,cardioversion, and/or pacing along with sensing functions, or whetherthe device is configured to detect and/or delivery drug/gene therapy orperform other therapeutic or diagnostic functions.

Device 20 can be proportioned to be passed into the vasculature and tobe anchored within the vasculature of the patient with minimalobstruction to blood flow. Suitable sites for introduction of device 20into the body can include, but are not limited to, the venous systemusing access through the right or left femoral vein or the right or leftsubclavian vein. In an alternate embodiment, the intravascularimplantable device may be configured for use in the arterial system.

For purposes of describing the present invention, the various portionsof the device 20 will be referenced to the location of those portions,the proximal portion 22, the middle portion 26 and the distal portion 24relative to the introduction site in the femoral vein. It will beunderstood, however, that if an alternate access site were used tointroduce the device 20, such as the subclavian veins, the variousportions 22, 24 and 26 of the device 20 would be referenced relative tothe inferior/superior location of the device 20 within the vascularsystem in the torso of a patient.

In one embodiment, the device 20 can have a streamlined maximum crosssectional diameter which can be in the range of 3-15 mm or less, with amaximum cross-sectional diameter of 3-8 mm or less in one embodiment.The cross-sectional area of device 20 in the transverse direction (i.e.transecting the longitudinal axis) can be as small as possible whilestill accommodating the required components. This area can be in therange of approximately 79 mm² or less, in the range of approximately 40mm² or less, or between 12.5-40 mm², depending upon the embodimentand/or application.

In one embodiment, the cross-section of device 20 (i.e., transecting thelongitudinal axis) may have a circular cross-section, although othercross-sections including crescent, flattened, or ellipticalcross-sections may also be used. It can be highly desirable to providethe device with a smooth continuous contour so as to avoid voids orrecesses that could encourage thrombus formation on the device. It canalso be desirable to provide for a circular cross-section to aid inremoval or explantation of the device that more easily permits thedevice to be torqued or rotated during the removal or explantation tobreak free of any thrombosis or clotting that may have occurred.

In one embodiment, the exterior surface of device 20 includes anelectrically insulative material, layer or coating such as ePTFE. Forexample, it may be desirable to provide a coating that isanti-thrombogenic (e.g., perfluorocarbon coatings applied usingsupercritical carbon dioxide) so as to prevent thrombus formation ondevice 20. It may also be beneficial that the coating haveanti-proliferative properties so as to minimize endothelialization orcellular in growth, since minimizing growth into or onto device 20 willhelp minimize vascular trauma when the device is explanted. The coatingmay thus also be one which elutes anti-thrombogenic compositions (e.g.,heparin sulfate) and/or compositions that inhibit cellular in growthand/or immunosuppressive agents. If the housing of device 20 isconductive, this layer or coating may be selectively applied or removedto leave an exposed electrode region on the surface of the housing wherenecessary, such as depicted in FIGS. 2A-2B and 3A-3B.

In one embodiment, the housing of device 20, or portions thereof, haveform factors designed to meet certain exterior boundary requirements.For example, an exterior boundary requirement may be a specifiedexterior geometry (such as a cylindrical or other suitable round shape),within certain dimensional tolerances. The housing according to thisembodiment may also have an enclosure thickness specification. Forexample, a particular cylindrical housing may have a 10 mm outerdiameter (OD) boundary specified with a tolerance, for example, of +/−5%tolerance, and a minimum wall thickness requirement of, for example, 1mm.

Given the minimal space allowed for components, the components mustthemselves be dimensioned to fit within the constraints of theenclosure. With reference to the above example of the 10 mm +/−5% ODwith minimum 1 mm walls, the components (including their interconnectingwiring) must fit within the form factor having a transverse dimension of10-10(0.05)-1, or 8.5 mm. It is desirable to arrange the componentswithin device 20 so as to make efficient use of the available space. Thesize and dimensions for the inductive element that can be achievedaccording to aspects of the invention provide additional flexibility inthe selection or design of these components since the inductive elementdesign can deliver desired performance characteristics within aspace-efficient volume, leaving relatively more volume available for thecomponents.

Examples of devices having space efficient arrangements of theircontents are shown in FIGS. 2A, 2B, 3A, and 3B. One example isidentified by reference numeral 20 a in FIG. 2A. One embodiment ofdevice 20 a includes one or more elongate housings or enclosures 32shown in cross-section in FIG. 2A to allow the components housed withinit to be seen. In one embodiment, enclosure 32 is a rigid or semi-rigidhousing optionally formed of a material that is conductive,biocompatible, capable of sterilization and capable of hermeticallysealing the components contained within the enclosure 32. One example ofsuch a material is titanium, although other materials may also be used.

Within enclosure 32 are the electronic components 34 that governoperation of the device 20 a. For example, in the FIG. 2A embodiment,some components are associated with delivery of a defibrillation pulsevia a lead 28, whereas other components are associated with the sensingfunction performed using sensing electrodes on the defibrillation leador on a separate lead 28. Isolating high voltage components from sensingcircuitry components may be desirable if electromagnetic interference(EMI) generated incidental to operation of the high voltage circuitrymight interfere with performance of the sensing circuitry. Isolation maybe achieved by increasing the physical separation between potentiallyinterfering and susceptible components, by electric field shielding, bymagnetic field shielding, or by a combination thereof.

Device 20 a further includes an energy source, such as one or morebatteries 36, for supplying power to the device. In certain embodimentsof cardioverter/defibrillator devices, one or more high-voltagecapacitors are provided for storing an electrical charge to be deliveredto the lead(s) 28 and/or one or more exposed electrodes 40 on anexterior surface of enclosure 32. One ore more circuit interconnects 42can provide the electrical coupling between the electronic components34, one or more leads 28, electrode(s) 40, batteries 36 and capacitors38.

As shown in FIG. 2A, the components of device 20 a may be arranged inseries with one another to give device 20 a a streamlined profile.Because device 20 a is intended for implantation within the patient'svasculature, some flexibility is desired so as to allow the elongatedevice to be easily passed through the vasculature. Flexibility may beadded by segmenting device 20, such as by forming one or more breaks inenclosure 32, and by forming one or more hinge zones at each break. Thehinge zones thus form dynamic flexible zones that can bend relative tothe longitudinal axis of the device 20 a in response to passage and/orpositioning of device 20 a though curved regions of the vasculature.

A second example of an arrangement of components for the intravascularimplantable pacing device is identified by reference numeral 20 b andshown in FIGS. 3A and 3B. Many of the components are the same as thoseshown in the FIG. 2A embodiment and will not be discussed again inconnection with FIGS. 3A and 3B. This second embodiment differs from thefirst embodiment primarily in that the electronic components 34 may beincluded within a single area of the enclosure 32. Alternatively, thedevice 20 b may include one or more breaks and hinge zones dependingupon the components and desired anchoring location for device 20 b. Thisconfiguration may be used, for example, when device 20 is intended onlyfor performing pacing functions (and thus lacks the relatively noisycharging circuitry found in the defibrillation circuitry), or ifisolation of the type shown in the FIG. 3A embodiment is not necessaryto prevent noise from the charging circuit from interfering with thesensing circuits.

According to another embodiment of device 20, each segment may beseparately enclosed by its own titanium (or other suitable material)enclosure in the form of containers. The components within thecontainers may be electrically connected by flexible circuit connects ,for example. In one embodiment, the containers are connected using aflexible material such as silicone rubber filler to form hinge zones.According to another embodiment, flexible device 20 includes one or morerigid enclosures or containers 32 used to contain electronic components34 to be implanted inside the vasculature of a patient and having thehinge zones formed of a bellows arrangement 48. Containers 32 can be ofany appropriate shape, cross-section, and length, but in this exampleare shown to have a cylindrical shape with a diameter of approximately3-15 mm and a length of approximately 20 mm to 75 mm. Containers 32 canbe used to house electromechanical parts or assemblies to formsophisticated implantable devices such as defibrillators, pacemakers,and drug delivery systems. Any appropriate number of these containers 32can be combined using interconnecting bellows 48. Interconnectingmechanical bellows 48 can be used, to connect a number of rigidcontainers 32 in order to form a flexible device 20. For many devices,this will include an arrangement of at least three containers 32.

In one embodiment, the bellows 48 can be of any appropriate shape, butcan have a shape similar in cross-section to the cross-section of thecontainer, in order to prevent the occurrence of edges or ridges thatcan give rise to problems such as the formation of blood clots in thevasculature. The bellows can be made of a biocompatible material similarto the containers. Any coatings used for electrically insulating thecontainers and/or making the containers more hemo-dynamically compatiblealso can be used with the bellows.

In addition to the ability of the bellows 48 to bend away from thecentral or long axis of device 20, the bellows 48 also allow forflexibility along the central axis of the device. The ability to flexalong the central axis provides shock absorption in the long axis aswell as 3-dimensional flexing. Shock absorption can help to protectdevice 20 and internal components during the implant process byminimizing the motion of the implanted device. Further, shock absorptioncan provide a 1:1 torque ratio for steering during the implant process.The shock absorption also can help during the life of device 20, as thenatural movement of the body of a patient can induce some stress on thedevice 20.

Referring again to FIG. 2A, electronic components that are associatedwith the delivery of defibrillation pulses include a voltage convertercircuit for converting the relatively low battery voltage to arelatively high electrotherapy voltage. One example of a relatively lowbattery voltage is a voltage less than about 20 volts. One example of arelatively high electrotherapy voltage is a voltage of about 50 volts ormore. In one example embodiment, the battery voltage is on the order of10 volts, and a maximum defibrillation voltage is on the order of700-1,000 volts. Generally speaking, embodiments of the voltageconverter can provide voltage boost on the order of about 5 to 300 timesthe voltage input to the converter. For instance, in one embodiment inwhich a 3 V battery is used as the energy storage for powering a boostcircuit that outputs defibrillation pulses at 1,000 V, the voltage boostis a factor of 333.

A voltage converter circuit that is of a switching mode type can be usedto produce the high-voltage output, which is, in turn, used to chargeone or more high-voltage capacitors situated at the output of thevoltage converter circuit. In some embodiments, the voltage convertercircuit is capable of charging the high-voltage capacitor(s) to store atleast 5 joules of energy is not more than 30 seconds. For example, inone embodiment, the voltage converter circuit can charge a high-voltagecapacitor to store about 30 joules in under 10 seconds. The energystored in the high-voltage capacitors is ultimately applied to thepatient during administration of the electrotherapy.

FIGS. 4A-4D and FIGS. 5A-5G are schematic diagrams illustrating variousexamples of known power converter circuit topologies. These topologiesare well-known by persons of ordinary skill in the relevant art, whowill appreciate that while these topologies themselves can not achievethe levels of performance and efficiency as taught by the presentinvention, variations of these topologies made in accordance with theteachings of the present invention may be used within the spirit andscope of the invention.

FIG. 4A illustrates a basic boost converter topology. The boostconverter of FIG. 4A utilizes a single inductor indicated at L1 to storeenergy in each cycle of switch SW. When switch SW closes, inductor L1 isenergized and develops a self-induced magnetic field. When switch SWopens, the voltage at the L1-SW-D1 node is boosted as the magnetic fieldin inductor L1 collapses. The associated current passes through blockingdiode D1 and charges energy storage capacitor C_(out) to a voltagegreater than input voltage V_(in).

FIG. 4B illustrates a flyback converter topology. The flyback converterutilizes transformer T1 as an energy storage device as well as a step-uptransformer. When switch SW is closed, the primary coil of transformerT1 is energized in similar fashion to inductor L1 of FIG. 4A. Whenswitch SW opens, the voltage across the primary coil is reversed andboosted due to the collapsing magnetic field in the primary. Thechanging voltages of the primary coil are magnetically coupled to thesecondary coil, which typically has a greater number of windings tofurther step-up the voltage on the secondary side. A typical turns ratiofor IID defibrillator applications in certain embodiments is Np:Ns ofabout 1:15, where Np is the number of primary turns and Ns is the numberof secondary turns. The high voltage across the secondary coil isrectified by the diode and stored in capacitor C_(OUT).

FIG. 4C illustrates a single ended primary inductance converter(“SEPIC”), which offers certain advantages over other power convertertopologies. For instance, the SEPIC converter offers an advantage of notrequiring significant energy storage in the transformer. Since most ofthe energy in a transformer is stored in its gap, this reduces the gaplength requirement for the transformer. Battery voltage (from the LiSVObattery, for example) is applied at VIN and the switching element isswitched at a fixed frequency and a duty cycle that is varied accordingto feedback of battery current into the power converter and outputvoltage. Voltage from the output of the step up transformer (T1) isrectified by the diode D1 to generate output voltage on COUT. Thecapacitance indicated at C_(OUT) represents the high voltage outputcapacitors.

FIG. 4D illustrates a variation of the SEPIC converter of FIG. 4C. TheSEPIC topology of FIG. 4D has an additional inductive component (L1).The additional inductor L1 can be implemented either discretely, or canbe magnetically coupled with the high voltage transformer into a singlemagnetic structure, as depicted in FIG. 4D.

FIG. 4E illustrates a Cuk converter topology. A Cuk converter comprisestwo inductors, L1 and L2, two capacitors, C1 and C_(out), switch SW, anddiode D1. Capacitor C is used to transfer energy and is connectedalternately to the input and to the output of the converter via thecommutation of the transistor and the diode. The two inductors L1 and L2are used to convert, respectively, the input voltage source (Vi) and theoutput voltage at capacitor C_(out) into current sources. Similarly tothe voltage converter circuits described above, the ratio of outputvoltage to input voltage is related to the duty cycling of switch SW.Optionally, inductors L1 and L2 can be magnetically coupled as indicatedT1*. In this arrangement, inductors L1 and L2 may be wound on a singlecore.

FIGS. 5A-5G also illustrate various magnetic core geometries that areknown in the art. Various E-shaped cores are depicted in FIGS. 5A-5D.FIG. 5A illustrates a classical E-core. The center leg's cross-sectionis generally larger than that of either peripheral leg, typically by afactor of two. In this geometry, the magnetic flux density is generallyuniform throughout the core, provided that the coil or coils are woundaround the center leg.

FIG. 5B illustrates an EFD core, in which the center leg is narrower inone dimension but wider in an orthogonal dimension. This type ofgeometry facilitates lower-profile inductive elements. FIG. 5Cillustrates an ER core, in which the center leg has a roundcross-section.

FIG. 5D illustrates an EP core, in which a generally cylindrical centerleg is partially surrounded by core material for the magnetic fluxreturn path. Between the center leg and surrounding portion is space inwhich the coil or coils would be situated. FIG. 5E illustrates a potcore which, like the EP core, has a center leg at least partiallysurrounded by core material with space therebetween for situating thecoil(s).

FIG. 5F is a diagram illustrating an inductor or transformer assemblyusing a two-piece core construction. Coil(s) 502 is wound around bobbin504, which is placed such that coil(s) 502 is positioned around thecenter leg of E core 506. An I-shaped core 508 is secured to the openend of E core 506 using clip 510. E core 506 and I core 508 positionedin this way produce a structure in the form of “EI” in which there ismagnetic material to guide a closed flux path through the center leg ofE core 506 and through the center of coil(s) 502, and returning throughthe peripheral legs of E core 506.

FIG. 5G illustrates another inductive element assembly utilizing atwo-part core. In FIG. 5G, a pair of opposing ER cores 512 a and 512 bis used. Coil(s) 502 are wound around bobbin 504, which has a lengththat is longer than the center leg of either ER core. When assembled,the pair of ER cores come together to complete the magnetic flux path.When an air gap is needed, the center leg of one or both E-type corescan be shorter than either of the peripheral legs. Similar structurescan be assembled using pot cores, different types of E cores, and othervariants thereof. As described above, these conventional geometries arenot well-suited for use in IIDs.

FIG. 6 is a diagram illustrating inductive element 600 according to oneembodiment of the invention. Inductive element 600 has a narrow formthat is well-suited for use inside the housing of an intra-vascularimplantable device such as device 20. In one exemplary embodiment, theouter surface of inductive element 600 generally conforms to an innersurface of a portion of enclosure 32. In this arrangement, a largeamount of interior volume of the portion of enclosure 32 that housesinductive element 600 is used for guiding magnetic flux. This provides arelatively larger inductance for inductive element 600, as compared withan inductive element of conventional geometry that would not occupy aslarge a proportion of interior volume space within a comparable portionof enclosure 23 as inductive element 600.

Inductive element 600 is assembled from a generally cylindrical magneticcore 602 having an outer surface 603, and a major length/situated alonglongitudinal reference axis x, and a generally round (e.g., circular,elliptical, etc.) cross-section situated along the transverse y-z plane.Magnetic core 602 is itself composed of two halves, lower half 602 a;and upper half 602 b. Situated within core halves 602 a and 602 b areone or more coils of wire 604. Although only a single coil is depictedfor the benefit of clarity, it is to be understood that a plurality ofcoils may be used to provide mutually-coupled transformer windings.

Each core half 602 a and 602 b has a mating surface 606 and a cut-awayportion 607. Cut-away portion 607 is defined by bottom surface 608,opposing walls 609, and post 610. Post 610 protrudes from bottom surface608 along reference axis z, and has a major length lp along longitudinalreference axis x, minor width wp along reference axis y, and protrudingheight hp along reference axis z. Post 610 also has a top surface 612that may be generally co-planar with mating surface 606.

In a related embodiment, top surface 612 is not co-planar with matingsurface 606; instead, top surface 612 is recessed relative to matingsurface 606. In this configuration, top surface 612 of core half 602 adoes not intimately contact the corresponding top surface of core half602 b when the core halves are joined. The resulting structure has anair gap between the opposing top surfaces 612. The height of post 610 ofeither or both core halves 602 a or 602 b may be designed to provide anair gap of a particular size to achieve desired magnetic properties forinductive element 600. As described above, the gap length determines theamount of energy that may be stored by inductive element 600, and alsoaffects the inductance of inductive element 600.

In another embodiment, core halves 602 a and 602 b are not identical.For example, bottom core half 602 a may have a post, while upper corehalf 602 b has no post. In this example embodiment, the post can have apost height that is taller than the height of opposing walls 609 in thez axis. In one related embodiment, the post height is about double theheight of opposing walls 609 in the z axis.

Coil 604 has a major length lc along reference axis x and a minor widthwe along reference axis y. Thus, coil 604 has elongate, or oblong,windings that define a correspondingly elongate, or oblong, loop areasituated longitudinally along the major axis of core 602. In oneembodiment, coil 604 is dimensioned such that, when inductive element600 is assembled, no winding of coil 604 protrudes beyond the outercylindrical periphery of core 602. As depicted in FIG. 6, coil 604 issituated to around, or in circumscribing fashion, to protrusion 610 inthe x-y reference plane. In one example embodiment, coil 604 ispre-formed with sufficient tolerance to permit sliding coil 604 overpost 610. In another embodiment, coil 604 is actually wound around post610.

In operation in this embodiment, the major magnetic flux componentproduced by current in coil 604 travels in a first direction along the zreference axis through protrusion 610. Minor magnetic flux components(summing to nearly equal the major magnetic flux component) return tocomplete the magnetic circuit through the remainder of core 602 (i.e.generally perpendicularly through mating surfaces 606 in the oppositedirection along the z reference axis. As can be seen from the geometryof inductive element 600, coil 604 produces the major flux componentalong an axis that is generally perpendicular to length l of the majoraxis of core 602 (i.e., in the y-z plane).

Generally speaking, the inductance of a coil of wire is a function ofthe relative permeability, the number of windings in the coil, the looparea defined by the coil, and the height dimension of the coilstructure. The inductance is directly proportional to the loop area, andinversely proportional to the height dimension of the coil structure.Therefore, in qualitative terms, a coil having a greater loop area and ashorter coil structure height will produce an element having arelatively greater inductance per unit length of wire comprising thecoil. Accordingly, in the constrained elongate form factor of animplantable intravascular device, the geometry of coil 604 providesdesirable inductive characteristics. The elongate or oblong shape of thewindings of coil 604 provide a relatively large loop area and arelatively small coil structure height. For example, in one embodiment,the major oblong loop dimension l_(c) of coil 604 is greater than theheight of coil 604 by a factor of 2.3. In a related embodiment, thesquare root of the loop area of coil 604 is greater than the height ofcoil 604 by a factor of 1.7.

In one embodiment, the loop area of coil 604 is greater than thecross-sectional area of inductive element 600 (the cross-section beingtaken in a plane perpendicular to the major length dimension l ofinductive element 600, such as in the y-z plane). In a relatedembodiment, the loop area of coil 604 is greater than thecross-sectional area of the form factor in which inductive element 600is enclosed.

By comparison, an inductive element having a solenoid-shaped coil (inwhich the coil structure length is similar to, or greater than, the looparea) such as the geometry of a coil structure used in a pot-type coreor in a core that is assembled with a wound bobbin, would requiresignificantly more wire length to achieve the same inductance as that ofsimilarly-dimensioned coil 604. This increased amount of wirecorresponds to a greater electrical resistance of the inductive elementand, consequently, reduced operating efficiency as an energy storageelement.

The geometry of core 602 provides further advantages relative toconventional pot cores when dimensioned to fit in the form factor ofIIDs. For example, core 602 provides a shorter flux path and a greatercross-sectional area for the magnetic flux than does the conventionalpot core. In one example embodiment, the total length of the closedmagnetic flux path is less than the length l of core 602. This type ofmagnetic circuit geometry of core 602 advantageously has less magneticreluctance, and thus more inductance per unit of core volume as comparedagainst the pot core geometry.

Comparing the core geometry of core 602 against conventional E- orC-type cores, core 602 is optimized to operate in the IID form factor.Thus, core 602 has more magnetic material for a greater cross-sectionalarea for the magnetic flux than does a similarly-dimensioned E- orC-type core. In one example embodiment, the sum of the areas of surfaces606 and 612 is greater than the area defined by the outer boundary ofthe cross-section of inductive element 600 (the cross-section beingtaken in a plane perpendicular to the major length dimension l ofinductive element 600, such as in the y-z plane). In a relatedembodiment, the sum of the areas of surfaces 606 and 612 is greater thanthe cross-sectional area of the form factor enclosing inductive element600. In another embodiment, the cross-sectional area of post 610, suchas the area of surface 612 alone, for example, is greater than the areadefined by the outer boundary of the cross-section of inductive element600.

In another related embodiment, the sum of the loop area of coil 604 andthe cross-sectional area of the portion of core 602 that is co-planarwith coil 604 and carries the returning magnetic flux, exceeds the areadefined by the outer boundary of the cross-section of inductive element600. In a related embodiment, the total area of the cross-section of themagnetic flux forward and return paths through core 602 is greater thanthe outer boundary of the cross-section of inductive element 600.

FIG. 7A is a diagram of inductive element 700 according to anotherembodiment. Inductive element 700 includes a generally cylindrical core702 having lower half 702 a and upper half 702 b, and coil 704. Coil 704is substantially similar to coil 604 of FIG. 6 in that both coils have agenerally elongate shape with a major dimension of the coil situatedalong the major dimension of the corresponding cylindrical core.

A portion of each core half 702 a and 702 b has a cross-sectional shapethat generally resembles the Cyrillic character “

” (Unicode character 0x042D). Each core half 702 a and 702 b has amating surface 706 that extends along the major length l′ of inductiveelement 700, and cavity 707 defined by generally cylindrical interiorsurface 708, and post 710 that protrudes from interior surface 708. Whencore halves 702 a and 702 b are interfaced with one another to produce acore assembly, the interfaced core halves define a pair of opposingcavities having a D-shaped cross-section and a length along the majordimension of core 702.

The structure and geometry of the core 702 of inductive element 700 canbe further described as a cylindrical shell having length l′, and innerand outer diameters defined by outer cylindrical surface 703 and innercylindrical surface 708. Post 710 substantially bridges the diameterover a portion of the length. Post 710 may fully bridge the diameter inan embodiment without an air gap in the post. Otherwise, where an airgap is desired, post 710 mostly bridge the diameter, save for the airgap.

Coil 704 (and optionally, additional coils) are assembled to fit incavity 707 and to be situated around, i.e., in circumscribing fashion,post 710. In one embodiment coil 704 does not protrude beyond the endsof core 702 (i.e., beyond the l′ dimension). Post 710 has a top surface712 that, like top surface 612, may be co-planar, or recessed, relativeto mating surface 706. Thus, assembled inductive element 700 may or maynot have an air gap. In one embodiment, mating surface 706 has a surfacearea that is equal to the surface area of top surface 712, such that thereturn path for the magnetic flux through the core has the samereluctance as the forward path. In a related embodiment, the sum of thesurface areas of surfaces 706 and 712 is greater than the area definedby the outer boundary of the cross-section of inductive element 700 (thecross-section taken perpendicularly to length l′). In another relatedembodiment, mating surface 706, alone, has a surface area greater thanthe area defined by the outer boundary of the cross-section of inductiveelement 700.

FIG. 7B is a cross-sectional view illustrating assembled inductiveelement 700. Core halves 702 a and 702 b are positioned together suchthat mating surfaces 706 are in intimate contact. Top surfaces 712 ofposts 710′, as depicted, are recessed relative to mating surfaces 706,producing air gap 720. Coil 704 is situated around, or in circumscribingfashion, about posts 710′ within cavity 707.

Operation of inductive element 700 is similar to that of inductiveelement 600. Current in coil 704 produces magnetic flux in core 702. Amajor component of the flux passes through protrusion 710. The returnflux path is distributed through the remainder of core 702. The enclosedgeometry of core 702 along the cylindrical wall provides additionalmagnetic shielding compared to that of core 602. In a relatedembodiment, the ends of core 702 are also closed to thereby virtuallyeliminate any field fringing effects occurring beyond the boundary ofthe core. This embodiment is, in a sense, a combination of core 602(having closed ends) and core 702 (having a closed cylindrical wall).

FIGS. 8 and 9 illustrate the magnetic flux density throughout each ofcores 602 and 702, respectively, based on computer-aided simulationresults. A comparison of the model of FIG. 9 against that of FIG. 8suggests core 702 provides a more uniform magnetic flux densitythroughout its volume than core 602. This may be explained qualitativelyfrom the fact that core 702 provides a greater surface area for themagnetic flux return path, and a shorter overall magnetic flux path.Additionally, in core 702, points along the return path are generallymore equidistant from the forward magnetic flux path compared with thoseof core 602. Thus, core 702 provides a magnetic circuit geometry withless reluctance and greater magnetic flux density uniformity than core602.

For any of the voltage converter circuits described herein, as well asfor other power circuits utilizing an inductive element according to theinvention, the inductive element can maximize converter circuitperformance and efficiency in view of the substantial geometricconstraints. More generally, in power converters that utilize mutuallycoupled coils, such as the certain Cuk, SEPIC or flyback topologies, forexample, the multiple coils can be accommodated by embodiments of theinductive element of the invention. Persons skilled in the relevant artswill appreciate that the inductive elements of the invention can beconstructed using known techniques and materials, such as, for example,from powdered ferrite stock. A variety of magnetic permeability rangesfor the core material may be used for different applications.

Embodiments of the invention enable certain operational performancemetrics to be achieved in the confined geometry of IIDs that otherwisewould not be attainable using conventional inductive elements in thepower converter circuitry. For instance, an IID defibrillator accordingto one embodiment, which has a diameter of less than 15 mm, and in oneembodiment less than about 8 mm, utilizes a Cuk or SEPIC power convertercircuit having an inductive element of a type described above. Thispower converter circuit can convert a battery voltage into anelectrotherapy voltage that is least ten times greater than the batteryvoltage, and output energy at that voltage at a rate of at least 1 Wwith an operating efficiency of at least 60% when the battery powersource is fully charged.

FIGS. 10 and 10A-10B is a diagram illustrating a power converter circuitand a portion of a control system for the power converter according toone example embodiment. The power converter topology in this example isa SEPIC power converter circuit in which inductor L1 is mutually coupledto transformer T1. Inductor L1 and transformer T1 are formed as amulti-winding inductive element having a geometry according to theembodiments described above. In this example, all of the windings aresituated around a common core.

Transistor Q1 operates in a switching mode that periodically energizesinductor L1 and the primary winding of transformer T1. The currentthrough the primary winding of transformer T1 is sensed and fed to thecontrol circuit as illustrated. Also, the output voltage HV is sensedand fed to the control circuitry. The output voltage is controlled byvarying the duty cycling of the drive signal to switching transistor Q1.

By sensing both, the output voltage, and the current through the primarywinding of transformer T1, the power converter of this example can bedynamically controlled to adjust its operating conditions so as toprovide the desired output at the best possible efficiency under thecircumstances. The circumstances may vary due to internal or externalevents. For instance, the battery voltage tends to drop as the battery'senergy is consumed over its life. In one embodiment, the control circuitadjusts operation of the power converter to accommodate this event.

In the embodiment illustrated in FIGS. 10 and 10A-10B, the functionalblocks depicted on the left-hand side of the Chip Boundary areimplemented in an application-specific integrated circuit (ASIC). Thecircuit portion on the right-hand side of the Chip Boundary isimplemented using discrete electronic components. In a relatedembodiment, groups of resistors, such as the six resistors used tocondition the current sense signal, are implemented using a resistornetwork such as a thin-film resistor network on a common substrate. Thistype of arrangement advantageously provides well-matched resistorshaving similar temperature coefficients and similar heating duringoperation.

The present invention may be embodied in other specific forms withoutdeparting from the spirit of the essential attributes thereof. Forexample, aspects of the invention are not limited to use exclusively inimplantable defibrillator devices. Other types of devices having a smallform factor and utilizing an inductive element may also benefit fromthese aspects of the invention. For example, implantable drug deliverydevices, electrostimulation devices, patient monitoring and datacommunication devices, and the like, may utilize one or more inductiveelements according to the invention.

Additionally, the invention is not necessarily limited to powerconverter circuits. Inductive elements according to aspects of theinvention may be utilized in other types of circuits and for a varietyof other functions such as, for example, for filtering, matching signalimpedance, and the like. Therefore, the illustrated embodiments shouldbe considered in all respects as illustrative and not restrictive,reference being made to the appended claims rather than to the foregoingdescription to indicate the scope of the invention. For purposes ofinterpreting the claims for the present invention, it is expresslyintended that the provisions of Section 112, sixth paragraph of 35U.S.C. are not to be invoked unless the specific terms “means for” or“step for” are recited in a claim.

1. An implantable intravascular medical device comprising: a structurethat defines a form factor having an elongate geometry including a formfactor length and a cross-section defined perpendicularly to the formfactor length; and a circuit situated within the form factor andincluding an inductive element; wherein the inductive element includes:a core of magnetic material having a core length and a corecross-section defined perpendicularly to the length; and a coil having aplurality of windings that define a loop area; wherein a portion of thecore is situated in the loop area such that the coil, when energized,produces a magnetic flux in the core along a forward path and a returnpath; and wherein a sum of a total cross-sectional area of the magneticflux in the forward path and a total cross-sectional area of themagnetic flux in the return path is greater than an area of the corecross-section.
 2. The implantable intravascular medical device of claim1, wherein the sum of a total cross-sectional area of the magnetic fluxin the forward path and the total cross-sectional area of the magneticflux in the return path is greater than an area of the cross-section ofthe form factor.
 3. The implantable intravascular medical device ofclaim 1, wherein the loop area is greater than at least one areaselected from the group consisting of: an area of the corecross-section, and an area of the cross-section of the form factor. 4.The implantable intravascular medical device of claim 1, wherein: theimplantable intravascular device includes a defibrillator and thecircuit is a power converter circuit; the form factor is generallycylindrical and has a generally circular cross-section, and thestructure that defines the form factor includes a housing having agenerally cylindrical outer surface and an inner surface that defines atleast a portion of the form factor, wherein the housing includes agenerally hermetic barrier; the inductive element is a transformerhaving a plurality of mutually-coupled coils that are entirely arrangedwithin a boundary defined by the outer surface of the inductive element;the core has a generally cylindrical outer boundary and a post having anoblong cross-sectional boundary with a major oblong dimension situatedlengthwise along the generally cylindrical outer boundary, the posthaving a post height situated radially in relation to the generallycylindrical outer boundary, and the coils being situated around thepost; and the core comprises a pair of core halves, wherein when thecore is assembled from the pair of core halves, the core halvesinterface at respective mating surfaces, and the post comprises a pairof post portions, each post portion corresponding to one of the corehalves and arranged such that the post includes an air gap.
 5. Aninductive element, comprising: a core of magnetic material having a corelength and a generally cylindrical outer boundary; and at least one coilhaving a plurality of windings that define a loop area; wherein aportion of the core is situated in the loop area such that the at leastone coil, when energized, produces a closed magnetic flux along a fluxpath through the core, wherein a length of the flux path is less thanthe core length.
 6. The inductive element of claim 5, wherein: theinductive element is a transformer having a plurality ofmutually-coupled coils that are entirely arranged within the outerboundary; the core includes a post having an oblong cross-sectionalboundary with a major oblong dimension situated lengthwise along thegenerally cylindrical outer boundary, the post having a post heightsituated radially in relation to the generally cylindrical outerboundary, and the coils being situated around the post; and the corecomprises a pair of core halves, wherein when the core is assembled fromthe pair of core halves, the core halves interface at respective matingsurfaces, and the post comprises a pair of post portions, each postportion corresponding to one of the core halves and arranged such thatthe post includes an air gap.